The invention relates generally to a transfer case for incorporation into a four wheel drive vehicle and more specifically to a transfer case having a planetary gear speed reduction assembly for providing high and low speed range outputs and a plurality of clutches for selectively engaging the direct and reduced speed output of the planetary gear assembly to a pair of output shafts. The transfer case may be controlled by a direct or target shift control.
The advantages of four wheel drive in light trucks, sport utility vehicles, off road vehicles and passenger cars are generally acknowledged. Improved vehicle stability and control extends the vehicle safety envelope when road conditions are wet, snowy or icy, for example, and extends the terrain capabilities in off road applications.
Every signpost does not point toward four wheel drive, however. Because of their complexity and increased frictional losses, fuel economy of a vehicle operating in four wheel drive will generally be lower than that of the same vehicle operating in two wheel drive or of a comparable two wheel drive vehicle.
Such increased fuel consumption is also the result of additional weight of four wheel drive systems. The weight increase may be significant if the system is intended for full time operation due to the necessary greater durability of the components whereas the weight penalty may be somewhat less if the four wheel drive components are intended only for limited, part-time use.
While most proponents of four wheel drive acknowledge that the need for and benefits of four wheel drive systems are infrequent and thus that four wheel drive systems which primarily utilize two driving wheels and which may be shifted into four wheel drive by the vehicle operator or by a control system satisfies most four wheel drive demands, such proponents will then also acknowledge that the design of such intermittent four wheel drive systems focuses attention on the mechanical components which effect the shift from two wheel drive to four wheel drive and, if it is an automatically engaging system, its control system.
Part-time four wheel drive trains and vehicles exhibit diverse operational features. For example, whereas certain drive systems and vehicles may be shifted from two wheel drive to four wheel drive at speed, that is, on-the-fly, other must be at a full stop before such shifts are achieved. Likewise, certain vehicle drive train configurations permit simple disconnection of the four wheel drive system and return to two wheel drive system at highway speeds whereas other drive trains require that the vehicle must be stopped or stopped and backed up several feet to properly disengage the system. Obviously, on-the-fly transition between two and four wheel drive is a highly desirable feature.
The operational distinctions between systems can be equally great with regard to the shift between low and high gears of the transfer case. Again, certain designs require that the vehicle be stopped, that it be travelling below a certain speed or be subject to other operational constraints. A design which allows the freedom of all reasonable low to high shifts not only provides increased user flexibility but improved durability and serviceability as well, inasmuch as actions which might constitute operator error and thus damage one vehicle drive system in one configuration are accepted as a normal, safe and appropriate command by another, more flexible system.
Another consideration of manually (driver) selected part-time four wheel drive systems is the driver/vehicle control interface. Three driving modes are offered by the typical system: two wheel drive (high gear); four wheel drive, high gear and four wheel drive, low gear. Conventionally, two momentary switches provide control pulses to a controller module which commands appropriate sequencing and action of, for example, the locking hubs and the transfer case gear range selection and clutch components. A first switch is designated four wheel drive, high gear and shifts the transfer case and vehicle into this driving mode. A second switch selects four wheel drive, low gear. The controller may also inhibit shifts in response to various vehicle conditions such as excessive speed.
Two wheel drive mode is the initial or default condition. To engage four wheel drive, high gear, the four wheel drive switch is depressed momentarily. To return to two wheel drive, the four wheel drive, high gear switch is again momentarily depressed. To achieve four wheel drive, low gear, the four wheel drive low gear switch is depressed once. To return to two wheel drive from four wheel drive low gear, the four wheel drive low gear switch must first be momentarily depressed which shifts the drive system to four wheel drive high gear. Then the four wheel drive high gear switch must be momentarily depressed which returns the drive system to two wheel drive. It is apparent that a simpler, more streamlined shift into and out of four wheel drive, low range is desirable.
The configuration and interaction of the mechanical and electronic components which contemporary four wheel drive systems include is the focus of much development effort. Much development effort generally is directed to what may be characterized as transition states, that is, the components, features and operation of the vehicle drive system as it shifts between two wheel and four wheel drive and back and also shifts between high and low gear ranges if the vehicle is so equipped.
A review of four wheel drive line and transfer case prior art technology reveals diverse approaches which provide commensurately distinct operational features. Said review also reveals that improvements in such drive systems and components are both desirable and possible.